Discrimination learning as a function of stimulus location along an auditory intensity continuum

Eight groups of rats were trained on an auditory intensity discrimination in which the discriminative stimuli were separated by 10 decibels (db). Four pairs of stimuli were selected from different regions along a 60-100 db (SPL) intensity continuum. Counterpart groups were trained on each stimulus pair, with the relative intensity positions of the reinforced stimulus (S(D)) and the non-reinforced stimulus (S(Delta)) reversed for the two groups. Discrimination acquisition curves were compared to determine whether stimuli separated by equal logarithmic units were of comparable "difficulty", and to determine the relative effectiveness of an S(D) serving as the more versus less intense member of a stimulus pair. It was concluded that: (1) When S(D) is the more intense, auditory intensities of constant logarithmic separation are graded in "difficulty" along the intensity continuum; high intensity discriminative stimuli are most readily discriminated. When S(Delta) is the more intense, this graded effect is not evident. (2) For a given continuum location, discrimination is inferior when S(Delta) is the more intense. This effect is most pronounced at the high intensity end of the continuum and is chiefly attributable to differences in the rate of S(Delta) responding.